Hybrid electric vehicles (HEV's) utilize a combination of an internal combustion engine with an electric motor to provide the power needed to propel a vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine. Clutches may be used to control the power flow and during transient vehicle operations, such as pulling up the engine or the electric motor, to provide smooth operation for the driver.
For example, the engine may be shutdown in an HEV during times that the engine operates inefficiently, or is not otherwise needed to propel the vehicle. In these situations, the electric motor is used to provide all of the power needed to propel the vehicle. When the driver power demand increases such that the electric motor can no longer provide enough power to meet the demand, if the battery state of charge (SOC) drops below a certain level, or another vehicle system requires engine operation, the engine may need to start to provide additional power for the vehicle. Depending on the state of the vehicle, it may be desirable to start the engine using various control sequences with the powertrain, i.e. the engine, motor, transmission, etc., to meet the vehicle and driver requirements at the time.